Dual waveguide mode source having control means for adjusting the relative amplitudesof two modes



June 1967 JAMES E. WEBB 3,32

ADMINISTRATOR OF THE NATIONAL AERONAUTICS AND SPACE ADMINISTRATION DUALWAVEGUIDE MODE SOURCE HAVING CONTROL MEANS FOR ADJUSTING THE RELATIVEAMPLITUDES OF TWO MODES Filed Dec. 29, 1964 FlG.i

FIG. 2

SOURCE OF TE iNVENTfiR H6. 3 ARTHUR C. LUDWIG ATTORNEY United StatesPatent 3,324,423 DUAL WAVEGUIDE MODE SOURQE HAVING CGNTROL MEANS FORADJUSTING THE RELATIVE AMPLITUDES OF TWO MGDES James E. Webb,Administrator of the National Aeronautics and Space Administration, withrespect to an invention of Arthur C. Ludwig, South Pasadena, Calif.

Fiied Dec. 29, 1964, Ser. No. 422,995 8 Claims. (Ci. 33321) Theinvention described herein was made in the performance of work under aNASA contract and is subject to the provisions of Section 305 of theNational Aeronautics and Space Act of 1958, Public Law 85-568 (72 Stat.435; 42 U.S.C. 2457).

This invention relates to microwave devices and, more particularly, toan improved waveguide mode generator.

Various microwave transmitting and receiving systems have been developedto track and/or communicate with long range missiles and other objectsin space. In all such systems, antennas are used to direct the microwaveenergy into space, and/or collect the received energy therefrom. Each ofthe antennas usually includes a reflecting dish and a feed systemthrough which the microwave energy is supplied to the reflecting dish,or received therefrom.

In order to improve a tracking, and/ or communication with the objectsin space, it is important to be able to control the shape of the beam ofmicrowave energy which the reflecting dish of the antenna eitherreflects into space or receives therefrom. Beam shaping is generallyaccomplished in whole or in part by adjusting a feed system to have acertain specified field distribution over its surface or aperture. Oneuseful method of accomplishing this is to adjust the relative phase andamplitudes of the various modes in which microwave energy will propagatethrough the horn of the feed system. The amplitudes and phaserelationships of these various modes are controlled to insure that theshape of the beam radiated by the feed and then reflected by the antennais of the desired configuration and magnitude.

In order to control the relative characteristics of the various modes ofenergy, microwave guiding and transmission systems have been developed.Generally such systems comprise a plurality of stages through which themicrowave energy is guided. These stages include various discontinuitiesat which secondary or higher order modes of microwave energy are excitedfrom the primary or dominant mode. In all prior art mode generators,used in beam shaping applications, the power of the higher order modesproduced as a result of the discontinuities is always less than thepower of the dominant mode. Thus, the power ratio between the power ofthe dominant and higher order modes is greater than one. Yet in beamshaping applications, it is often necessary to have more energy in thesecondary, or higher order modes, than in the dominant mode. Although acertain amount of energy in the dominant mode is necessary, in generalunless special steps are taken, there will be an excess amount of energyin the dominant mode. However, none of the prior art waveguide modegenerators used in beam shaping applications are adapted to convenientlycontrol the relative amounts of energy or amplitudes of the dominant andsecondary, or higher order modes.

Accordingly, it is an object of the present invention to provide awaveguide mode generator wherein the relative power of various modes maybe controlled.

It is another object of the present invention to provide a waveguidemode generator wherein a higher order mode generated from a dominantmode is accentuated while the dominant mode is diminished in power.

A further object of the present invention is to provide a waveguide modegenerator wherein the relative power Patented June 6, 1967 between adominant mode and a higher order mode generated therefrom may becontrollably adjusted.

Still a further object of the present invention is to provide awaveguide mode generator which is designed to accentuate the higherorder mode TE and diminish the dominant mode TE with the relative powerbetween the two modes being controlled by these relative positions of acontrolling member within the waveguide mode generator.

These and other objects of the invention are achieved in a waveguidemode generator in which a discontinuity is present so that a dominantmode supplied to the generator, is excited to generate higher ordermodes. The dimensions of the generator are selected to insure that adesired one of the higher order modes is generated from the inputenergy. In addition, the generator includes a control element which isselectively positionable within the waveguide mode generator so that thepower of the dominant mode used as the input energy may be controlledwith respect to the power of the higher order mode generated therein.The control element may be positioned to provide power ratios whereinthe power of the dominant mode is less than that of the power of thehigher order mode. Thus, the generator is capable of providing microwaveenergy wherein the energy of a secondary mode is accentuated withrespect to the energy in a dominant mode from which the secondary orhigher order mode had been produced.

The novel features that are considered characteristic of this inventionare set forth with particularity in the appended claims. The inventionitself both as to its organization and method of operation, as well asadditional objects and advantages thereof, will best be understood fromthe following description when read in connection with the accompanyingdrawings, in which:

FIGURE 1 is an excitation pattern diagram of energy in mode TE FIGURE 2is an excitation pattern diagram of energy in mode TE and FIGURE 3 is aside elevational view of the waveguide mode generator of the presentinvention.

The teachings of the present invention will be described in conjunctionwith a waveguide mode generatordesigned so as to accentuate the higherorder or secondary mode T5 and diminish the dominant mode TE from whichthe secondary mode TE is excited. Reference is now made to FIGURES l and2 which are excitation pattern diagrams of microwave energy in modes TEand TE respectively, in the E plane, propagating in cylindricalwaveguide sections of diameter B. When comparing the patterns of the twomodes, it is seen that within the pattern of the mode TE a patternidentical to that of mode TE propagates within a circular area ofdiameter A, which is shown in FIGURE 1 by a dashed line 12.

On the basis of mathematical analysis of microwaves, the diameter A maybe determined whenever the diameter B is known. The interrelationship ofthe patterns of the mode TE and TE in pattern diameters B and Arespectively, is used in designing the mode generator of the presentinvention. A waveguide section of diameter A is incorporated to carry orprovide the mode TE Such section is abruptly changed in cross-section toa section of diameter B, so that at the discontinuity mode TE isstrongly excited.

Reference is now made to FIGURE 3 which is a side elevationalview of acylindrical mode generator 15 constructed in accordance with theteachings of the present invention. As seen therein, the generator 15comprises an input section 20, a horn-shaped output section 25, and anexcitation section 30 interposed therebetween. The cylindrically-shapedinput section 20 has an internal diameter A, and an input opening 20athrough which energy in mode TE may be supplied from a source ofmicrowave energy in mode TE designated by numeral 35. The energy in modeTE propagates through the input section 20 towards the other endthereof, at which the excitation section 30 with an internal diameter Bis connected. Due to the difference in diameters of the two sections, adiscontinuity surface 32 is created.

Assuming that the microwave energy from the source 35 is in a pure TEmode, it is apparent to those familiar with the art that thediscontinuity between sections 20 and 30 will cause secondary or highermodes of the dominant mode TE to be excited therein. Diameter B, forreasons to be explained hereafter, is chosen such that the guidewavelength of mode TE is twice the guide wave length of mode TE By sochoosing the diameter of section 30, modes of order TM and higher willbe beyond cutoff in the excitation section and therefore will notpropagate through it without severe attenuation. Assuming the incidenceof a pure TE mode, due to the circular symmetry of the generator, onlymodes In, where 12:1, 2, 4 etc. will be excited. But since modes of theorder TM and higher will be beyond cutoff only modes TE TM and TE willbe excited and propagate through section 30. The TM mode is also excitedand controlled by the generator. However, the control of modes TM and TEis not independent, with only one mode being adjustable to a specifiedvalue relative to the TE mode. The generator used as the example toexplain the teachings of the present invention is primarily related tocontrol the relative amplitudes of modes TE and TE excited therefrom.

In the absence of additional means, the generator so far described issimilar to prior art generators, in that a dominant mode is propagatedthrough a waveguide discontinuity so that secondary higher order modesmay be generated. As previously stated, in prior art mode generators,the power of the second modes is always less than that of the dominantmode. However, in the novel generator of the present invention, as shownin FIGURE 3, additional control means are provided in order to controlthe power relationship or power ratio between that of the dominant andsecondary modes.

As seen in FIGURE 3, the generator of the present invention furtherincludes a cylindrically-shaped control element 40 which is insertablewithin the input section 20. The wall thickness of the control element40 is small so that the small change in the effective diameter A of theinput section 20 does not adversely affect the energy in mode TEpropagating therethrough. The control element 40 is adjustablypositioned within the input section 20 so that only a selected portionthereof, designated in FIGURE 3 by numeral 45, may extend beyond theinput section 20 into the excitation control section 30.

From FIGURE 3, it is seen that the energy in mode TE propagating throughsection 20 does not enter the excitation section 30 at the point ofdiscontinuity therebetween. Rather, the energy enters the excitationsection at the end face of the control element 40. Therein, modes TE andTE in a waveguide section of a diameter B are excited. Some of thisenergy will propagate towards the horn-shaped section 25. However, someof the energy will be reflected towards the surface of discontinuitybetween the sections 20 and 30. Therefrom, the energy will be reflectedback towards the horn-shaped section 25. Thus, the energy directedtowards the horn-shaped section comprises energy propagating directly ina forward direction from the element 40, as well as energy reflectedback from the surface of discontinuity between the sections 20 and 30.The two wave fronts will reinforce or subtract from the total energy ineach one of the modes depending on the guide wavelength of each mode andthe dimension designated by arrow 45. Namely, the extent to which thecontrol element 40 is inserted into the excitation section 30 willcontrol the relative amplitudes and power of the modes propagatingtherethrough.

As previously assumed, the diameter B is chosen so that the guidewavelength of the secondary modes T15 is twice the guide wavelength ofmode TE Thus, if the dimension designated by arrow 45 is adjusted to beone-quarter wavelength of the mode TE and one-half wavelength of themode TE the reflected back wave reinforces for TE and cancels for modeTE Consequently, the amplitude or power of the dominant mode TE will begreatly reduced with respect to the amplitude or power of the secondarymode excited therefrom.

The relative amplitudes of the two modes can be adjusted by properlypositioning the control element 40 so that the dimension designated byarrow 45 is equal to a predetermined portion of the guide wavelength ofeither of the modes. The distance from the end face of the controlelement 40 to the end of the excitation section 30, designated in FIGURE3 by arrow 50, is controlled to provide the desired relative phaserelationship between the dominant mode TE and the secondary mode TEexcited therein.

From the foregoing, it is seen that the present invention provides amode generator wherein a secondary mode such as TE is not only excitedfrom a dominant mode such as TE but the relative amplitudes of suchmodes are conveniently controllable. Such control is accomplished bylimiting the distance between the discontinuity surface 32 and the endof the control section 40 through which energy propagates, namely, thedistance indicated by arrow 45 to be related to the guide wavelengths ofthe modes to be controlled.

The invention has been described in conjunction with a cylindricalwaveguide mode generator wherein the relative amplitudes of a dominantmode and a simple secondary mode excited therefrom are controlled.However, it is apparent to those familiar with the art that theteachings disclosed herein are not limited to a cylindrical waveguidemode generator. The control element 40 may similarly be incorporated inother generators such as square or rectangular. Also, the generator neednot be limited to exciting and controlling modes TE and TE Rather, thegenerator of the invention may be used to control the relativeamplitudes of other modes. For example, the relative amplitudes of modesTM and TM described hereinbefore, the only requirement being that themodes be interrelated.

There has accordingly been shown and described herein, a novel anduseful mode generator wherein the relative amplitudes of a mode excitedfrom a primary mode are controllable. Such control is extremely usefulin the shaping of beams, in particular, and in the radiation ofmicrowave energy in general.

What is claimed is:

1. In a waveguide mode generator wherein energy supplied in a dominantmode is excited to produce energy in a secondary order mode relatedthereto the improvement comprising a first cylindrical waveguide sectionto which said energy is supplied in said dominant mode, having adiameter sufliciently large to enable the energy in said dominant modeto propagate therethrough; a second cylindrical waveguide section havinga diameter greater than the diameter of said first section coupled tosaid first section so as to produce a discontinuity therebetween; and acylindrical control waveguide element positioned within said firstwaveguide section and extending into said second waveguide section saidcylindrical control waveguide element having a diameter large enough forenabling the energy in said dominant mode to propagate into said secondsection through said element at a predetermined distance from saiddiscontinuity to be excited therein so as to generate said secondarymode said distance being a function of the guide wavelengths of saiddominant and secondary modes, so as to control the relative power of theenergy in said modes propagating out of said second waveguide section.

2. In a waveguide mode generator wherein energy supplied in a dominantmode is excited to produce energy in a secondary mode related thereto,the improvement comprising a first waveguide section having a firstdiameter sufficiently large to enable energy in said secondary mode tobe propagated therethrough, but small enough to suppress other higherorder modes; a second waveguide section having a second diameter smallerand related to said first diameter, said second diameter beingsutficient to propagate energy in said dominant mode therethrough,including means for coupling said second section to said first sectionso as to form a discontinuity therebetween; and a control waveguidesection adjustably extendable into said first waveguide section fromsaid second waveguide section for controlling the position from saiddiscontinuity at which energy propagating through said second waveguidesection enters said first waveguide section so as to control therelative amplitudes of energy in said dominant mode and said secondarymode generated therein.

3. In a waveguide mode generator as recited in claim 2 wherein each ofsaid first, second and control waveguide sections are cylindrical inshape, said first diameter being sufiiciently large to enable energy inmode TE to be excited therein from energy in mode TE propagating throughsaid second waveguide section, and wherein the portion from saiddiscontinuity at which energy in said TE mode enters into said firstwaveguide section is related to the guide wavelengths of said TE and T13modes.

4. In combination with a waveguide mode generator having a discontinuitybetween first and second waveguide sections so that at least onesecondary mode is excited from a dominant mode propagating through saidfirst section into said second section, a control element selectivelypositionable within said first section with a portion thereof extendinginto said second section so as to control the relative amplitudes ofsaid dominant and secondary modes in said second section, as a functionof the relative position of said control element in said second sectionfrom said discontinuity.

5. In combination with a waveguide mode generator having a firstcylindrical waveguide section through which energy in a first dominantmode propagates and a second cylindrical waveguide section coupled tosaid first section so as to form a discontinuity for generating aselected secondary mode related to said dominant mode, a cylindricalcontrol member adjustably positioned in said first section, having aselected portion thereof extend into said second waveguide section to anextent related to the guide wavelengths of said dominant and secondarymode so as to affect the relative amplitudes of the dominant andsecondary modes propagating in said second cylindrical waveguidesection.

6. In a waveguide mode generator wherein energy supplied in a dominantmode is excited to produce energy in said dominant mode and in at leastone secondary mode related thereto, the arrangement comprising a sourceof energy in a dominant mode; a first waveguide section having a firstinternal dimension; means for conmeeting said source of energy to saidfirst waveguide section for supplying said energy thereto, said firstdimension of said first section being sufficiently large to enableenergy in said dominant mode to propagate through said first section; asecond waveguide section having a second dimension greater than saidfirst dimension for exciting said dominant and secondary modes connectedto said first section so as to form a waveguide discontinuity surfacetherebetween; a control waveguide section having a third dimensionmounted within said first section and having a portion along thelongitudinal axis thereof extending into said second section forcontrolling the position within said second section at which energy insaid dominant mode propagating through said first section and controlsection enters said second section, so as to control the amplituderelationship between said dominant and secondary modes excited in saidsecond section.

7. In a waveguide mode generator wherein energy supplied in a dominantmode is excited to produce energy in said dominant mode and in at leastone secondary mode related thereto, the arrangement comprising a sourceof energy in a dominant mode; a first cylindrical waveguide sectionhaving a first diameter; means for connecting said source of energy tosaid first waveguide section for supplying said energy thereto, saidfirst diam eter of said first section being sufficiently large to enableenergy in said dominant mode to propagate through said first section; asecond cylindrical waveguide section having a second diameter greaterthan said first diameter for exciting said dominant and secondary modesconnected to said first section so as to form a Waveguide discontinuitysurface therebetween; a cylindrical control waveguide section having athird diameter mounted within said first section and having a portionalong the longitudinal axis thereof extending into said second sectionat which energy in said dominant mode propagating through said firstsection and control section enters said second section, so as to controlthe amplitude relationship between said dominant and secondary modesexcited in said second section.

8. In a waveguide mode generator as recited in claim 7 wherein saiddominant mode is TE and said second diameter is large enough so thatmodes TM and TE are excited in said second section, said portion of saidcontrol section extending into said second section being related to theguide wavelengths of modes TE and TE so as to control the relativeamplitudes of the energy in modes TE and TE which are excited in saidsecond section.

References Cited UNITED STATES PATENTS 2,283,935 5/1942 King 343786 X2,632,804 3/1953 Jouguet 333-21 2,950,452 8/1960 Marcatili 33398 X3,268,902 8/1966 Turrin 343772 3,305,870 2/1967 Webb 333-21 X ELILIEBERMAN, Primary Examiner.

NUSSBAUM, Assistant Examiner,

4. IN COMBINATION WITH A WAVEGUIDE MODE GENERATOR HAVING A DISCONTINUITYBETWEEN FIRST AND SECOND WAVEGUIDE SECTIONS SO THAT AT LEAST ONESECONDARY MODE IS EXCITED FROM A DOMINANT MODE PROPAGATING THROUGH SAIDFIRST SECTION INTO SAID SECOND SECTION, A CONTROL ELEMENT SELECTIVELYPOSITIONABLE WITHIN SAID FIRST SECTION WITH A PORTION THEREOF EXTENDINGINTO SAID SECOND SECTION SO AS TO CONTROL THE RELATIVE AMPLITUDES OFSAID DOMINANT AND SECONDARY MODES IN SAID SECOND SECTION, AS A FUNCTIONOF THE RELATIVE POSITION OF SAID CONTROL ELEMENT IN SAID SECOND SECTIONFROM SAID DISCONTINUITY.